The proposed study will examine the neurological and endocrine mechanisms supporting environmentally adaptive buffering of stress induced behavioral and physiological alterations in two subspecies of white-crowned sparrows. These populations are of special interest because several previous studies have revealed substantial differences between them in terms of both their behavioral and hormonal responses to stress across the breeding season. The proposed study will search for differences between these subspecies during the early and late phases of the breeding season in relation to glucocorticoid and mineralocorticoid receptors in areas of the brain including the hippocampus, hypothalamus, and neocortex, as well as in peripheral tissue. In addition, we will also compare these two subspecies in terms of the sensitivity of corticosterone response to ACTH, AVT, CRF, and MT during both the early and late breeding season. This study will provide insight into the location and function of mechanisms enabling stress response buffering, as well as a better understanding of the flexibility of such adaptations.